Atrial fibrillations have been observed after termination of ventricular arrhythmias by cardioversion, as described in an article entitled "Comparative Efficacy of Transvenous Cardioversion and Pacing in Patients with Sustained Ventricular Tachycardia: A Prospective, Randomized, Crossover Study," by Saksena et al., in Circulation 72, No. 1, pages 153-160, 1985. Termination with transvenous cardioversion was followed by occurrences of atrial fibrillation, atrial flutter and sinus tachycardia. See, also, an article entitled "Transvenous Cardioversion and Defibrillation of Ventricular Tachyarrhythmias: Current Status and Future Directions," by Saksena et al., in PACE, Vol 8, pages 715-731, 1985. In this study, the incidence of supraventricular tachyarrhythmias after transvenous cardioversion was substantial. Such problems continue to exist.
In patients receiving cardioversion shocks using prior art devices, it has been observed in some cases that such post-therapy arrhythmias have been attributable to the shock being delivered during the vulnerable zone of the atrium. In those cases where atrial arrhythmias occur at a sufficiently fast rate, there is a likelihood of this arrhythmia being detected as VT/VF, resulting in the patient receiving an unnecessary shock to the ventricles of the patient's heart. This highlights the need for an implantable device having the capability of effectively sensing atrial fibrillation, and having the ability both to switch to an atrial cardioversion configuration and to synchronize atrial cardioversion shocks to the ventricular rhythm of the patient, thereby to successfully treat atrial arrhythmias such as atrial fibrillation and atrial flutter.
An article entitled "Atrial Fibrillation and Embolic Complications in Paced Patients," by H. Langenfeld et al., in PACE, Vol. 11, pages 1667-1672, 1985, shows the high incidence of atrial fibrillation in patients with VVI pacemakers. This is said to be attributable to irritation of the atrial rhythm caused by retrograde conduction. Thus, certain combined implantable defibrillator/pacemakers not only may contribute to the cause of the problem of atrial fibrillation, but also have been found to lack the facilities to deal successfully with atrial arrhythmias. Thus, there is a need for implantable devices capable of successfully treating atrial arrhythmias.
U.S. Pat. No. 3,857,398 to Rubin describes a combined pacer/defibrillator. This device performs either a pacing or a defibrillation function, depending on the detection of a VT/VF. If a VT/VF is detected, the device is switched to the defibrillating mode. After a period of time to charge the capacitor, a defibrillation shock is delivered to the patient.
A multiprogrammable, telemetric, implantable defibrillator is disclosed in the co-pending U.S. patent application Ser. No. 576,178 of Norma L. Gilli et al., entitled "Reconfirmation Prior to Shock for Implantable Defibrillation." The device contains a bradycardia support system as well as a high energy defibrillation shock system to revert ventricular tachycardias to normal sinus rhythm. On reconfirmation of the presence of a tachycardia, a shock is delivered to the ventricle of a patient at a predetermined time or when the desired energy level is reached. This device is not capable of delivering atrial cardioversion in order to alleviate the condition of atrial fibrillation.
U.S. Pat. No. 4,572,191 to Mirowski et al. describes an atrial cardioverting device that is externally driven by either the physician or the patient. The detection of an arrhythmia requires the patient to recognize it, which is disadvantageous for a number of reasons including medical clinic visits, carrying expensive equipment around, or just the failure to recognize a tachycardia. Furthermore, the device is incapable of automatic detection and reconfirmation, as well as failing in the capacity to defibrillate the ventricles of a patient in the event an atrial shock accelerates the rhythm into VT/VF. This patent also describes the delivery of cardioversion shocks using a single pacing lead. A single pacing lead has been found, by recent research, to be inadequate in effectively discharging a cardioversion shock. Significantly, the main reason for this problem is that pacing leads possess a very low surface area, giving rise to a high impedance at the area of discharge. Aside from the device using too much power, the patient is subjected to a risk of tissue damage at the electrode interface.
Another problem exists with the Mirowski et al. device as there are no provisions therein for sensing R-waves and for pacing the patient's ventricle. Therefore the device is unable to synchronize the cardioversion shock to the ventricle. As a result, the unsynchronized atrial cardioversions delivered to a patient may cause VF, resulting in a further hazardous situation.
A further disadvantage of the above device is that it is turned off except when it is externally engaged by a magnet that allows the power source to charge. Thus the device is inadequate as an automatic implantable therapeutic device. Also, since the device is not capable of being instituted within a pacemaker defibrillator system, it does not have provisions for allowing bradycardia pacing, single or dual chamber antitachycardia pacing and defibrillation therapy to the ventricle. Therefore, aside from failing to adequately cardiovert the atrium successfully and automatically, the device fails as an all round therapeutic medical device offering a variety of treatments to the patient.
It is an object of the present invention to provide an improved implantable device for the automatic detection of atrial arrhythmias and for providing atrial cardioversion therapy therefor.
It is also an object of the invention to achieve effective cardioversion of atrial fibrillation with minimal power drain, and to prevent tissue damage due to high voltages being discharged over a small area, such as results from the use of a single pacing electrode.
It is a further object of the invention to provide a device having at least two defibrillation endocardial electrodes or other suitable electrodes of surface area substantially greater than that of a normal pacing lead, and of substantially lower impedance than the latter, and including at least one subcutaneous patch, which device does not require a magnet or any other external manual switching system for turning on the power source to charge its capacitor.
Another object of the invention is to provide an improved device which is on call at all times, and which is capable of being incorporated within an implantable automatic pacemaker defibrillator/cardioverter having the ability to provide defibrillation therapy to a patient's ventricle, as well as antitachycardia pacing therapy and bradycardia support pacing to either or both chambers of the heart when required.
It is yet another object of the invention to provide an automatic implantable device capable of delivering low energy cardioversion therapy into the atrium in order to improve the health and safety of patients by returning atrial arrhythmias to normal rhythms, thereby obviating the need for unnecessary high energy shocks.
A still further object of the invention is to minimize complications associated with atrial arrhythmias, since patients who have experienced atrial fibrillation have been reported to have a higher mortality rate due to embolism development.
An additional object of the invention is to provide a device which has the ability to achieve atrial cardioversion, either with R-wave synchronization or during a device-initiated ventricular refractory period, to insure that the vulnerable zone of the ventricle is avoided, thereby minimizing post-shock arrhythmias attributable to shock delivery during the ventricular vulnerable zone and degeneration of the arrhythmia into VT/VF.
It is a further object of the invention to provide an atrial cardioversion device having a plurality of electrode configurations, and having the ability to switch from one electrode configuration to another prior to the delivery of a shock or between deliveries of consecutive shocks.
Further objects and advantages of this invention will become apparent as the following description proceeds.